_sd-common.h: avoid parsing errors with Coverity
The commit
1070d271fa8fa553d57dd5f74dd1e3f60732d0b9 which was supposed
too fix this does not seem to take effect any more. We get again 34%
compilation success rate while scanning systemd itself. Moreover, the
installed header file breaks compilation of programs that include it:
"/usr/include/systemd/_sd-common.h", line 23: error #35: #error directive: "Do
not include _sd-common.h directly; it is a private header."
# error "Do not include _sd-common.h directly; it is a private header."
^